Hurricanes and Tornadoes Hurricanes and Tornadoes HC > FAQ > Hurricanes and Tornadoes Hurricanes and Tornadoes 1. How are tropical cyclones different from tornadoes ? While both tropical cyclones and tornadoes are atmospheric vortices, they have little in common. Tornadoes have diameters on the scale of 100s of meters and are produced from a single convective storm (i.e. a thunderstorm or cumulonimbus). A tropical cyclone, however, has a diameter on the scale of 100s of kilometers and is comprised of several to dozens of convective storms. Additionally, while tornadoes require substantial vertical shear of the horizontal winds (i.e. change of wind speed and/or direction with height) to provide ideal conditions for tornado genesis, tropical cyclones require very low values (less than 10 m/s [20 kt, 23 mph]) of tropospheric vertical shear in order to form and grow. These vertical shear values are indicative of the horizontal temperature fields for each phenomenon: tornadoes are produced in regions of large temperature gradient, while tropical cyclones are generated in regions of near zero horizontal temperature gradient.

Tornadoes are primarily an over-land phenomena as solar heating of the land surface usually contributes toward the development of the thunderstorm that spawns the vortex (though over-water tornadoes, or waterspouts do occurr). In contrast, tropical cyclones are purely an oceanic phenomena - they die over land due to the loss of a moisture source. Lastly, tropical cyclones have a lifetime that is measured in days, while tornadoes typically last on the scale of minutes. For more information on tornadoes you can go to the Storm Prediction Center's FAQ maintained by Roger Edwards. Tropical cyclones at landfall often provide the conditions necessary for tornado formation. As the tropical cyclone makes landfall and begins decaying, the winds at the surface die off quicker than the winds at, say, 850 mb. This sets up a fairly strong vertical wind shear that allows for the development of tornadoes, especially on the tropical cyclone's right side (with respect to the forward motion of the tropical cyclone). For the southern hemisphere, this would be a concern on the tropical cyclone's left side - due to the reverse spin of southern hemisphere storms. (Novlan and Gray 1974) . 2.

Why do tropical cyclones spawn tornadoes? Tropical cyclones spawn tornadoes when certain instability and vertical shear criteria are met, in a manner similar to other tornado-producing systems. However, in tropical cyclones, the vertical structure of the atmosphere differs somewhat from that most often seen in midlatitude systems. In particular, most of the thermal instability is found near or below 10,000 feet altitude, in contrast to midlatitude systems, where the instability maximizes typically above 20,000 feet. Because the instability in tropical cyclones is focused at low altitudes, the storm cells tend to be smaller and shallower than those usually found in most severe midlatitude systems. But because the vertical shear in tropical cyclones is also very strong at low altitudes, the combination of instability and shear can become favorable for the production of small supercell storms, which have an enhanced likelihood of spawning tornadoes compared to ordinary thunderstorm cells ( Novlan and Gray 1974 , Gentry 1983 , McCaul 1991 ). 3. What percentage of tropical cyclones spawn tornadoes ?

Almost all tropical cyclones making landfall in the United States spawn at least one tornado, provided enough of the tropical cyclones circulation moves over land. This implies that Gulf coast landfalling tropical cyclones are more likely to produce tornadoes than Atlantic coast tropical cyclones that sideswipe the coastline. The rate at which tropical cyclones produce waterspouts over the ocean is unknown, although Doppler radars have identified many cases where storm cell rotation suggestive of the presence of tornadoes was observed over water ( Novlan and Gray 1974 , Spratt et al. 1997 ). 4. What parts of a tropical cyclone are most favored for tornado formation ? In the northern hemisphere, the right-front quadrant (relative to tropical cyclone motion) is strongly favored. In the southern hemisphere, the left-front quadrant presumably is favored, although there is little research on this point. Most of the tornadoes form in outer rainbands some 50-200 miles from the tropical cyclone center, but some have been documented in the inner core, or even in the eyewall ( Novlan and Gray 1974 , McCaul et al. 1996 , Spratt et al. 1997 ).

5. How long after landfall are tropical cyclone tornadoes a threat ? Tropical cyclones may spawn tornadoes from a day or two prior to landfall to up to three days after landfall. Statistics show that most of the tornadoes occur on the day of landfall, or the next day. The most likely time for TC tornadoes is during daylight hours, although they can occur during the night, too ( Novlan and Gray 1974 , McCaul 1991 ) . Although statistically the largest number of tropical cyclone tornadoes occurs on the day of landfall, some of the biggest and most damaging outbreaks have taken place 1 or 2 days after landfall, as with Beulah in 1967, Danny in 1985, and Beryl in 1994. In the case of Florida, with its peninsular shape, many of the tornadoes occur as the outer rainbands reach the state, well prior to the landfall of the tropical cyclone center ( Hagemeyer and Hodanish 1995, Hagemeyer 1997 ). 6. Are tropical cyclone tornadoes weaker than midlatitude tornadoes ?

In general, it appears that TC tornadoes are somewhat weaker and briefer than midlatitude tornadoes. During the period 1948-1986, the percentage of TC tornadoes that reached F2 or greater intensity on the Fujita scale was 26% ( McCaul et al. 2004 ), while during a roughly comparable period (1950-1976), the corresponding percentage for all U.S. tornadoes was 36% (Kelly et al. 1978). In addition, there have been no F5-rated TC tornadoes since reliable records commenced in 1950, and only two F4s. There have, however, been numerous F3s, and some of these have caused many casualties and much damage. Of course, we cannot rule out the possibility that a future TC might spawn an F5 tornado ( Gentry 1983 , McCaul 1991 ). In Florida, in particular, the most significant tornadoes tend to occur with "hybrid" cyclones or tropical cyclones with some hybrid influence.

This usually means greater westerly shear in the storm environment which is believed to be favorable for stronger, longer-lasting tornadoes. Hurricane Agnes in June 1972 was a minimal category 1 hurricane with considerable hybrid influence and it produced the most F2 and greater tornadoes in a single day in Florida history ( Hagemeyer 1998, Hagemeyer and Spratt 2002 ). 7. What is the largest known outbreak of tropical cyclone tornadoes? 2004's Hurricane Ivan caused an outbreak of 117 tornadoes - with the bulk of the tornadoes on 17 September - which developed over a 3 day period in the United States, including 37 in Virginia, 25 in Georgia, 18 in Florida, 9 in Pennsylvania, 8 in Alabama, 7 in South Carolina, 4 in both Maryland and North Carolina, 3 in West Virginia, and 2 in Maryland. There were 26 tornadoes reported on 15 September, 32 tornadoes on 16 September, 57 tornadoes on 17 September, and 2 tornadoes (in Maryland) on 18 September.

At least 8 people were killed and 17 injured by the tornadoes. The previous record was during Hurricane Beulah, which spawned a reported 115 tornadoes in southeast Texas during the first several days after its landfall in September 1967 ( Orton 1970 ). These outbreaks of tornadoes from Ivan and Beulah represent two of the largest tornado outbreaks of any kind in the U. S. tornado climatology. It is difficult to predict which tropical cyclones will produce large tornado outbreaks, although there is some indication that the likelihood of a major outbreak increases as TC size and intensity increase. 8. What is the deadliest single TC-spawned tornado ? One of the tornadoes spawned in October 1964 by Hurricane Hilda killed 22 people in Larose, LA ( Novlan and Gray 1974 ). 9. What is the most damaging single TC-spawned tornado ? One of the tornadoes produced by Hurricane Allen in 1980 did about $100 million damage, in recent dollars, in the Austin, TX, area ( Gentry 1983 ). 10. Why are tropical cyclone tornadoes especially difficult to forecast?

TC tornadoes are often spawned by unusually small storm cells that may not appear particularly dangerous on weather radars, especially if the cells are located more than about 60 miles from the radar. In addition, these small storms often tend to produce little or no lightning or thunder, and may not look very threatening visually to the average person. Furthermore, the tornadoes are often obscured by rain, and the storm cells spawning them may move rapidly, leaving little time to take evasive action once the threat has been perceived. ( McCaul et al. 1996 , Spratt et al. 1997 ). 11. Which states are most vulnerable to TC tornadoes outbreaks ?

Historical records show that the largest and most intense TC tornado outbreaks have occurred in Texas (Hurricane Carla in 1961, Beulah in 1967, Allen in 1980, and Gilbert in 1988), Louisiana (Hurricane Audrey in 1957, Carla in 1961, Hilda in 1964, and Andrew in 1992), Mississippi(Hurricane Audrey in 1957 and Andrew in 1992), Alabama (Hurricane Audrey in 1957, Danny in 1985, Andrew in 1992, and Georges in 1998), South Carolina (Tropical Storm Beryl in 1994), and Florida (Hurricane Agnes in 1972, Opal in 1995, and Tropical Storm Josephine in 1996). The Gulf Coast states tend to have the most frequent and significant TC tornado events, partly because of their tendency to have at least one state fully exposed to the right-front quadrant of the tropical cyclone when landfall occurrs( McCaul 1991 ). 12.

How does the damage from hurricanes compare to tornadoes? Even though winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes typically cause much more damage individually and over a season. The strongest tornadoes - those of Fujita Tornado Damage Scale 4 and 5 - have estimated winds of 207 mph [333 kph] and higher, while the strongest hurricanes - those of Saffir-Simpson Hurricane Scale 4 and 5 - have winds of 131 mph [210 kph] and higher. Hurricanes in the continental U.S. cause on average about $3 billion per landfall and about $5 billion annually ( Pielke and Landsea 1998 ). The roughly 1000 tornadoes that impact the continental U.S.each year cause about ten times less - about $500 million in total ( Brooks and Doswell 2001 ). The top 30 most damaging hurricanes in the last 100 years (normalized to account for higher population, wealth and inflation) have each caused over $2.9 billion ( Jarrell et al. 2001 ).

In comparison, only the most damaging tornado in the last 100 years or so - if it hit today - would cause about $2.9 billion in damage: the May 1896 St. Louis tornado ( Brooks and Doswell 2001 ). Hurricanes tend to cause much more destruction than tornadoes because of their size, duration and variety of ways to damage items. The destructive circular eyewall in hurricanes (that surrounds the calm eye) can be tens of miles across, last hours and damage structures through storm surge, rainfall-caused flooding, as well as wind impacts. Tornadoes, in contrast, tend to be a mile or smaller in diameter, last for minutes and primarily cause damage from their extreme winds. References: Brooks, H. E., and C. A. Doswell, III, 2001: Normalized damage from major tornadoes in the United States: 1890-1999. Wea. Forecasting , 16, 168-176. Jarrell,J.D., M. Mayfield, E.N. Rappaport, and C.W.

Landsea, 2001: "The Deadliest, Costliest, and Most Intense United States Hurricanes from 1900 to 2000 (and other Frequently Requested Hurricane Facts)" NOAA Technical Memorandum NWS/TPC-1. Honolulu, Hawaii Weather Forecast Office 2525 Correa Rd, Ste 250 Honolulu, HI 96822 d The Saffir Simpson hurricane scale rates hurricanes with - BCOM - 3950 d. The Saffir-Simpson hurricane scale rates hurricanes with wind speeds of 114 to 131 knots as Category 4 storms. This preview has intentionally blurred sections. Sign up to view the full version. View Full Document CHAPTER 10 – TEST BANK  102 ANS: d REF: p. 265 TYPE: Ap DIF: 5 TOP: Learning What to Document NOT: You must document information that is not common knowledge.

The statement that is not common knowledge is “The Saffir-Simpson hurricane scale rates hurricanes with wind speeds of 114 to 131 knots as Category 4 storms.” AAC: Tier1—Ethics; Tier 2—Plagiarism AAC: Tier 1—Reflective Thinking; Tier 2—Analysis, Conclusion 26. Which of the following is the best advice about using direct quotations in a report? a. Beware of overusing quotations because you may appear as if you have no ideas of your own. b. If you use a direct quotation, don’t dilute its impact by summarizing it or introducing it in your own words. c. Place the direct quotation in quotation marks, but change a few words to show you have your own ideas. d. Direct quotations do not need to be placed in quotation marks if the audience is already familiar with the quote, such as, “The buck stops here.” ANS: a REF: p. 267 TYPE: Con DIF: 3 TOP: Knowing When and How to Quote NOT: You should beware of overusing quotations because readers may think that you have few ideas of your own.

AAC: Tier1—Ethics; Tier 2—Plagiarism AAC: Tier 1—Reflective Thinking; Tier 2—Conclusion 27. Tyler is writing a report that analyzes the GlobalCom, Inc., market share of cell phone subscriptions worldwide by region. He will probably organize his report a. chronologically by the date the cell phone subscriptions reached their maximums. b. around extensive secondary data to assure the reader that he conducted adequate research before he wrote the report. c. using a functional arrangement to show types of subscriptions globally . d. geographically, with sections representing each region’s market share . ANS: d REF: p. 268 TYPE: Ap DIF: 5 TOP: Organizational Strategies NOT: Since Tyler’s purpose is to analyze market share by region, he would most likely arrange his report geographically. AAC: Tier1—Communication; Tier 2—Patterns AAC: Tier 1—Reflective Thinking; Tier 2—Analysis, Conclusion 28. Which of the following is the best advice when using graphics in reports? a. Don’t overuse color because too much color can be distracting and confusing. b.

Good graphics communicate clearly and need no further explanation in the body of the report. c. Misleading graphics in a report if their inclusion was unintentional. d. All answer choices are good advice when incorporating graphics into reports. 103  CHAPTER 10 – TEST BANK ANS: a REF: p. 276 TYPE: Con DIF: 3 TOP: Incorporating Graphics in Reports NOT: The best advice is “Don276’t overuse color because too much color can be distracting and confusing.” AAC: Tier1—Technology; Tier 2—Data conversion AAC: Tier 1—Reflective Thinking; Tier 2—Conclusion 29. In her letter to policyholders, Min Yi needs a graphic depicting the procedure a policyholder follows when filing a claim. Which of these would be most appropriate? This preview has intentionally blurred sections. Sign up to view the full version. Hurricanes, Typhoons, Cyclones | UCAR - University Corporation for Atmospheric Research UCAR - University Corporation for Atmospheric Research Background on the science, people, and issues involved in hurricane research Frequently asked questions Hurricane links Why do hurricanes happen? Most hurricanes start life as areas of rough weather and thunderstorms in the tropics.

Many of these disturbances, or tropical waves, produce little more than heavy rain and gusty winds. But if a tropical wave succeeds in spinning into a complete circle of winds rotating around an area of low air pressure at its center, it's given the name tropical depression. When a depression's peak sustained winds reach 39 miles per hour (34 knots), it's called a tropical storm. As a tropical system strengthens, its winds spiral inward, concentrating moisture near the center. This spiraling, a result of Earth's rotation, can't happen near the equator. To benefit from the curving winds produced by the Coriolis effect , a storm needs to be at least 300 miles (500 kilometers) north or south of the equator. When a tropical storm maintains wind speeds of at least 74 miles per hour (65 knots), it's known as a hurricane, at least in North and Central America (see What's the difference between a hurricane and a typhoon or tropical cyclone? ). Each year, hundreds of tropical depressions spin up over warm waters worldwide. But more is needed for a depression to grow into one of Earth's most powerful storms.

Besides warm ocean temperatures, the depression needs lots of warm, moist air to feed on. Winds also matter. Strong differences in wind direction and strength at different heights in the atmosphere, known as wind shear, can pull a tropical system apart, dissipating its energy. So low wind shear is also essential to tropical storm formation. (But note that high wind shear plays a role in tornado formation, including tornadoes that form over land as a hurricane dissipates .) For more about the life cycle of tropical storms, from their birth as tropical depressions, to full-blown hurricanes or cyclones, to their ultimate demise, see Hurricanes & Tropical Cyclone Life Cycles (from UCAR COMET's Hurricane Strike!). back to top What's the difference between a hurricane and a typhoon or tropical cyclone? When a tropical disturbance organizes to the point where its sustained winds top 34 knots (39 miles per hour), it's known as a tropical cyclone . But various parts of the world use a variety of terms once a tropical cyclone packs winds of at least 65 knots (74 mph) in a particular " basin " (an oceanic region where these storms occur). Around North and Central America, they're called hurricanes.

The god of evil for the Carib people was named Hurican, according to the authors of Hurricane Strike! That's the source, with a slight twist in spelling, of the name used in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and Northeast Pacific Ocean. In the Northwest Pacific, the same powerful storms are called typhoons. In the Southeastern Indian and Southwest Pacific Oceans they're called severe tropical cyclones. In the North Indian Ocean, they're called severe cyclonic storms, while in the Southwest Indian Ocean, they simply keep the name tropical cyclone. Hurricanes that make Category 3 status on the Saffir-Simpson Hurricane Wind Scale (winds of at least 96 knots or 111 mph) are labeled intense hurricanes. If a typhoon hits 132 knots (150 mph), it becomes a supertyphoon. When a system weakens below hurricane strength, it is typically reclassified as a post-tropical cyclone. This was the case with Hurricane Sandy in 2012 just before landfall.

In order to reduce confusion, the National Weather Service introduced a new policy in 2013 in which hurricane warnings may be continued even after a storm has technically become post-tropical. back to top When is hurricane season? Hurricane season in the Atlantic Basin (including the Caribbean and the Gulf of Mexico) runs from June 1 through November 30, with most activity clustered from August to October. The typhoon and cyclone seasons follow their own patterns. In the Northeast Pacific, the official season runs from May 15 to November 30. In the Northwest Pacific, typhoons are most common from late June through December. The North Indian Ocean sees cyclones from April to December, with peaks in May and November. The Southwest Pacific and South Indian oceans (including the waters bordering Australia) get most of their activity from November to May. If the conditions are right, tropical cyclones can develop outside their official seasons, especially in the Northwest Pacific, where they occur year round. NOAA's Hurricane Research Division has more about seasonal timescales . back to top How big and how strong can hurricanes get? Whatever you call them, these monster storms are the most powerful atmospheric phenomena on Earth.

Hurricanes gather energy from water vapor in the atmosphere stretched for hundreds of thousands of square miles across the warm ocean water of the tropics. The storms themselves can bring heavy rains and gale-force winds blowing 38 to 73 miles per hour (62–117 kilometers per hour) across an area the size of Louisiana. Hurricane-force winds can extend 50 miles (80 km) or more from the storm center. How big? According to NOAA's National Hurricane Center , the average hurricane eye —the still center where pressure is lowest and air temperature aloft is highest—stretches 30 miles (48 km) across, with some growing as large as 120 miles (200 km) wide. The eye typically shrinks as a hurricane intensifies, sometimes narrowing to less than 10 miles (16 km) in width. Eventually, a new eye may form around the old one; hurricanes often weaken during this transition but can intensify again as the new eye contracts. How strong? Hurricane intensity is categorized on different scales around the world.

The strongest storms, equivalent to Category 5 on the Saffir-Simpson scale , have sustained winds that exceed 155 miles per hour (135 knots). In order to grow, hurricanes need plenty of warm, moist air to form showers and thunderstorms, along with winds that don't change direction much with height, which allows the central circulation to develop undisturbed. If these atmospheric conditions are right, then hurricane strength is dictated largely by the presence or absence of deep, warm ocean water (ideally 79°F or warmer [26°C or warmer]). As winds strengthen, more water evaporates, releasing energy stored in the warm seas. Using ocean data, scientists can estimate the maximum potential intensity (MPI), or the peak hurricane strength one might expect at a given location. Scientists are now exploring how best to calculate and predict MPI in computer models. Size does not determine intensity. Some of the most destructive hurricanes to hit the U.S. coastline, including Hurricane Andrew in 1992, extended over a relatively small area. Whatever their size or strength, long-lived hurricanes normally "spin down" after they come ashore. The land surface creates drag and the oceanic heat source disappears.

The high winds gradually weaken and spread over a larger area. Even as they decay, hurricanes can still bring damaging winds more than 100 miles inland. Also, the frictional effects on surface winds can help create the wind shear needed to spawn tornadoes. One hurricane can produce dozens of tornadoes as it moves ashore. But the worst hurricane damage is often the result of a storm surge that causes coastal flooding . Along parts of the Mississippi coast, the surge from Hurricane Katrina was 28 feet or more above mean sea level, putting it among the highest surges ever recorded in the United States. Large waves on top of a storm surge cause even more damage. Surge risk varies depending on the strength and structure of a given hurricane as well as geography and tidal cycles along the coast where it strikes. Although Superstorm Sandy fell just below hurricane strength several hours before its center reached southern New Jersey in October 2012, Sandy was an extremely large system, and it struck near high tide. As a result, the storm pushed huge amounts of water into the New York and New Jersey coastlines, producing a catastrophic surge that set records in several places. Originally, the Saffir-Simpson scale included typical storm surges expected for each category.

In the case of Sandy and several other recent hurricanes, storm surges have been higher than the levels once assigned to each Saffir-Simpson category. The categories have been revised so that they now refer only to wind speed, and the National Hurricane Center is moving toward new ways of depicting the threat from storm surge, including storm surge watches and warnings that will be distinct from hurricane watches and warnings, as described in this 2012 article . Storms that move inland often bring much-needed rain that farmers and water managers count on. Recent studies indicate that across parts of the U.S. Deep South, up to 15% of all warm-season rainfall typically comes from hurricanes and tropical cyclones. But if too much falls at once, the rain can quickly overwhelm stream and river beds, producing serious river flooding . back to top Can we control hurricanes? Even smaller hurricanes pack a mind-boggling amount of power. The heat energy released by a hurricane equals 50 to 200 trillion watts—or about the same amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes.

We are unlikely to come up with methods to control such overwhelming natural power for the foreseeable future—though that has not kept people from speculating about what it would take . Researchers instead focus on understanding every aspect of hurricane structure and behavior, with the hope their work will lead to better predictions of storm tracks and intensity so warnings can be issued to protect life and property. The societal side of the equation includes communication between forecasters and emergency managers who make decisions about when and where to call for evacuation from threatened areas and then get the word out with help from local public safety offices and the mass media. Researchers at NCAR are collaborating with colleagues elsewhere to address the human side of hurricane forecasts and warnings through the Collaborative Program on the Societal Impacts and Economic Benefits of Weather Information (SIP). Research Updates Studies show 63% of hurricane-related deaths occur inland. NCAR scientists have been using tropical storm winds, census data, and flood maps to help emergency managers identify the most vulnerable populations and plan their response. The resulting 2012 map reveals the surprising extent of risk . An overview in the Bulletin of the American Meteorological Society (August 2012) summarizes recent work by NCAR scientists and collaborators on the communication of hurricane risk.

back to top Hasn't the number of hurricanes been going down lately? On average there are about 70 to 110 named tropical cyclones per year across the world, including about 40 to 60 that reach hurricane strength. This range has held remarkably steady within the last 40 years. Within each basin , the numbers often vary more dramatically than the global average. In part this is due to ocean-atmosphere cycles such as El Niño and La Niña . Because they affect where showers and thunderstorms develop, these cycles can suppress hurricane activity in one basin while enhancing it in another. During El Niño years, the Atlantic tends to be less active than usual, while parts of the central and northeast Pacific are typically busier than usual. While the long-distance effects are not exactly opposite during La Niña events, there is a tendency for more activity in the Atlantic than the Pacific. Even though the total number of tropical cyclones around the world holds fairly steady, some years are more active than others. Scientists often use an index called Accumulated Cyclone Energy, or ACE , to measure the overall intensity of a given year’s cyclone activity.

ACE takes into account a storm’s peak winds at each six hours of its lifetime. After dipping to 30-year lows in the early 2010s, global ACE values have begun rising again ( see graph ). Of all the hurricanes that build over the North Atlantic and Gulf of Mexico each year, only a small fraction make it to the U.S. coastline at hurricane strength. As of early 2013, the nation had not seen a major landfall (Category 3 or stronger) since 1995. However, the total number of hurricanes swirling across the North Atlantic remains unusually high. back to top Are hurricanes striking in new places? In recent years, we’ve seen several hurricanes and tropical storms strike in unfamiliar places. This could be a result of improved monitoring, regional changes in ocean temperatures and upper-air circulation (perhaps linked to global warming in some cases), natural variability, or all of the above. The South Atlantic had been considered free of tropical cyclones—that is, until March 2004, when a mysterious storm later dubbed Hurricane Catarina made landfall in Brazil. In October 2005, Vince became the first tropical storm ever recorded in Spain.

And in the Arabian Sea, Gonu became a Category 5 in June 2007—that region’s strongest tropical cyclone on record. After weakening, Gonu brought unprecedented damage from rain, wind, and flooding to parts of Oman and Iran. Although the Bay of Bengal has seen the world's deadliest tropical cyclones on record, most of these strike India and Bangladesh, while the coast of Myanmar tends to experience only weaker cyclones. The catastrophic Cyclone Nargis , which struck Myanmar in May 2008, appears to be the first major cyclone (Category 3 strength or higher) ever recorded in the populous Irrawaddy Delta. More than 90,000 people died in the storm and its aftermath. The mid-Atlantic and New England can see intense hurricanes, but quite rarely. These are almost always moving north to northeast, often sideswiping the coastline. However, unlike any other hurricane in modern records, Hurricane Sandy moved west-northwest toward New Jersey, making landfall just after becoming post-tropical yet still delivering an unprecedented storm surge to the region. Is global warming affecting hurricanes? Are hurricanes getting worse as the global average temperature rises ? What can we expect in the future?

A growing amount of research is looking into these questions, and not all of the answers are in just yet. Here’s a summary of what research tells us so far. Oceans are warmer; more rainfall expected The bottom line: As summarized in 2007 in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change : There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases in tropical SSTs [sea-surface temperatures]. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. —IPCC Working Group I, Summary for Policymakers, 2007 Sea-surface temperatures across the tropics have risen along with global temperature over the last 100 years and are expected to warm further in the next century. All else being equal, warmer oceans can support stronger hurricanes. The extra water vapor evaporated by oceans in a warmer climate can be expected to boost rainfall from hurricanes by as much as 8% for every 1°C (1.8°F) of warming, according to Kevin Trenberth of NCAR ( view a PDF of this paper ).

Examining the record Several studies in the last decade have reported enhanced hurricane activity in some regions. For example: Roughly twice as many hurricanes are now reported in the Atlantic compared to a century ago. The rise, identified by Greg Holland (NCAR), occurred over three distinct periods associated with rises in sea-surface temperature (see NCAR news release ). In both the North Atlantic and North Pacific oceans, the duration of tropical cyclones as well as their strongest wind speeds have both increased by about 50% over the past 50 years, according to Kerry Emanuel, Massachusetts Institute of Technology (see MIT news release ). The number of Category 4 and 5 hurricanes worldwide nearly doubled from the early 1970s to the early 2000s, found a team from the Georgia Institute of Technology and NCAR (see NCAR news release ). Changes in observing techniques pose a major challenge when studying past hurricanes. Tropical cyclones have been routinely monitored by aircraft for 60 years or less, and by satellite for only 20 to 40 years, depending on the ocean.

In earlier years, many storms over the open ocean may have gone unobserved, as noted in a 2007 study by Christopher Landsea (NOAA, view a PDF of this paper ). Thus, scientists must take special care in analyzing global hurricane records prior to the 1970s. Studying the future The bottom line: According to the 2012 IPCC report Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation ( PDF ): Average tropical cyclone maximum wind speed is likely to increase, although increases may not occur in all ocean basins. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged. (Section 3.4.4) Computer modeling Because the inner core of a hurricane only spans a few miles, and the space between data points in a typical global climate model is wider than that, global models cannot yet produce realistic hurricanes. Scientists are using several techniques to get around this roadblock.

For example: A 2008 study (see NOAA news release ), led by Thomas Knutson at NOAA’s Geophysical Fluid Dynamics Laboratory, combined a higher-resolution regional model with IPCC projections for the late 21st century of global-scale atmosphere and ocean conditions. The team's technique indicated an average drop of 18% in the number of Atlantic hurricanes and a 2% rise in average intensity. One limitation to this study is that the model had difficulty producing hurricanes at Category 3 strength or greater. (Christopher Landsea discussed this issue in a 2007 article in EOS , as did Knutson et. al. in an online FAQ .) Using statistics on hurricane development, MIT’s Kerry Emanuel "seeded " a set of IPCC global projections with weak tropical waves, then tracked those that developed, using a more-detailed, hurricane-scale ocean-atmosphere model. His 2008 study (see MIT news release ) showed a tendency toward more and stronger tropical cyclones in the North Atlantic and Northwest Pacific, with fewer but more intense cyclones globally. However, there are substantial differences among the various IPCC projections used, plus a good deal of variability from decade to decade ( view a PDF of this paper ).

Several groups are attempting to produce global models that can directly depict hurricanes without an intervening small-scale model. The first such study, produced on Japan’s Earth Simulator computer and released in 2005, indicated more and stronger hurricanes in the Atlantic by later this century. Globally, it showed a 30% drop in the number of tropical cyclones, but a rise in the number and strength of the most intense hurricanes ( view a PDF of this paper ). Meanwhile, research to boost the resolution of computer models continues. At NCAR, researchers are experimenting with the global Community Climate System Model (see Global model goes local ), the Weather Research and Forecasting model (see Weather forecast goes global ) and idealized models of hurricane structure that reveal details never depicted before, such as fine-scale turbulence in a hurricane eyewall . Multidecadal patterns in the Atlantic Hurricane researcher William Gray (Colorado State University) and others have emphasized the role of a natural 20- to 40-year cycle in ocean temperature, dubbed the Atlantic Multidecadal Oscillation (see NOAA FAQ ), in shaping hurricane activity across the North Atlantic.

If only the AMO were considered, Atlantic hurricane counts would be expected to drop for several decades beginning in the 2010s or 2020s. However, analyses by other researchers, including Kevin Trenberth (see NCAR news release ), have examined the AMO influences in light of global warming changes in ocean temperatures to conclude that the AMO is only a minor factor. Instead, they argue, the recent uptick in Atlantic hurricane activity is more closely related to overall global warming and thus may continue for decades to come. Wind shear and hurricanes Some scientists are exploring how interactions among different parts of the tropics will unfold in a warmer climate. For example, Gabriel Vecchi (NOAA) and Brian Soden (University of Miami) have found that wind shear over the North Atlantic may increase in the coming century, with the western tropical Pacific warming even more than the tropical Atlantic (see American Geophysical Union news release ). This setup would help to inhibit Atlantic hurricanes. However, not all of the IPCC models examined by Vecchi and Soden show this pattern—a sign of the continued challenge in portraying tropical ocean circulation in global models.

Understanding hurricane structure and behavior Flying into the storms One of the best ways to figure out how hurricanes operate is to fly right into them. Each year, skilled pilots steer research aircraft as close as safety allows to storms forming in the North Atlantic and Pacific. The planes are part of NOAA's hurricane hunter fleet and other research operations supported by the U.S. Navy, Air Force, universities, and research labs. Improving forecasts close to shore In 2008 NOAA adopted a technique called VORTRAC to provide detailed 3-D views of an approaching hurricane every six minutes. The three-dimensional views help to determine whether the storm is gathering strength as it nears land (see NCAR news release ). VORTRAC (Vortex Objective Radar Tracking and Circulation) was developed by researchers at NCAR and the Naval Research Laboratory. The technique requires no major new hardware, but instead relies on the existing NOAA network of Doppler radars along the Southeast coast to closely monitor hurricane winds. About 20 of these radars are scattered along the Gulf and Atlantic coastlines from Texas to Maine. Each radar can measure winds blowing toward or away from it, but no single radar could provide a 3-D picture of hurricane winds before now.

NCAR scientist Wen-Chau Lee and his collaborators developed a series of mathematical formulas that combine data from a single radar with general knowledge of Atlantic hurricane structure in order to map the approaching system's winds in three dimensions. The technique also infers the barometric pressure in the eye of the hurricane, a very reliable index of its strength. Forecasters using VORTRAC can update information about a hurricane each time a NOAA Doppler radar scans the storm, which can be as often as about every six minutes. That could enable forecasters to monitor it for the critical 10-15 hours before landfall. Wringing moisture data out of GPS signals At UCAR’s COSMIC program , researchers have shown that computer models do a better job of predicting hurricanes when they’re fed extra data on how much moisture is in the air surrounding a developing storm. The data to help predict which potential hurricanes are most likely to develop come from SuomiNet , a UCAR-managed network of ground-based sensors that assess moisture by measuring tiny changes in GPS signals. Research Update Meteorologists have hypothesized that ventilation, the injection of cooler and drier air into the core of a tropical cyclone, can significantly lessen the cyclone's intensity.

To further understand this phenomenon , in 2012, NCAR scientist Brian Tang and colleague Kerry Emanuel (Massachusetts Institute of Technology) published work using a hurricane model to determine how much ventilation is needed to weaken a tropical cyclone and where this is most likely to happen within it. back to top High-resolution forecasting from weather models Forecasters at NOAA’s National Hurricane Center make use of a wide array of computer models, including one called Hurricane WRF. Adopted by NOAA in 2007 , Hurricane WRF was derived from the Weather Research and Forecasting model (WRF) and configured to reveal tropical storm activity in great detail. A multiagency effort, WRF is a next-generation computer model for weather prediction that can be used by both researchers and operational forecasters. See the roster of NOAA/NCEP model output to find Hurricane WRF forecasts of current storms. NCAR has also refined research-oriented versions of WRF over the last few years, using them to study hurricanes and other forms of severe weather.

To accurately depict the small but intense features within hurricanes, a special version of the Advanced Research WRF (ARW) sharpens the detail over targeted regions to 7.5 miles (12 kilometers) for forecasts out to 120 hours, with a resolution as fine as 0.8 mi (1.33 km) near hurricanes. Since 2005, NCAR's experimental forecasts of hurricane track and intensity have ranked among the most accurate of the computer models used by researchers and forecasters to predict the season's hurricanes. See the ARW modeling page for forecasts of current storms. During August–October 2013, NCAR will be conducting 3- to 7-day hurricane forecasts using the experimental Model for Prediction Across Scales. When run on high-performance supercomputers, MPAS can simulate weather processes around the globe in very fine detail, capturing the development and evolution of individual cloud systems—a long-time goal of weather researchers. MPAS will help pave the way toward more detailed simulations by linking tropical storm and hurricane activity with global atmospheric conditions. The new simulations will provide better intensity forecasts than regional models at roughly the same cost for computing.

Experimental accuracy This side-by-side animation of Hurricane Katrina's path across the Gulf of Mexico compares the actual radar observations (left) with NCAR's Advanced Research WRF experimental forecast, issued 62 hours before landfall. In both frames, narrow rainbands can be seen pinwheeling counterclockwise into the storm's core. The model resolution for this animation was 12 kilometers (7.5 miles). The radar vantage point is stationary, on the Gulf Coast, while the model's viewpoint follows the hurricane itself. Click here or on the image to launch the animation in a new window. Anatomy of a hurricane This 3-D illustration identifies four components of a hurricane: inflow, rainbands, eye and eyewall, and outflow. Warm, moist air enters the hurricane at low levels (large orange arrows), rises through intense updrafts in the eyewall (spiral at center), and departs the storm at high altitudes (large blue arrow). A weaker downdraft makes the eye itself relatively calm and clear. Rising air also helps create concentric rainbands (short orange arrows) outside the eyewall, with sinking air (short blue arrows) in between the rainbands.

Click here or on the image for an interactive version with more detail. (Illustration © COMET , UCAR.) News media terms of use * The eyewall replacement cycle - a key event in the life of a hurricane Eyewall replacement during Katrina The peak winds of over 100 miles per hour that buffeted New Orleans during Hurricane Katrina could have been much worse had the storm made landfall at a different moment in the cycle of its eyewall . Long-lived, intense hurricanes often go through an eyewall replacement cycle that takes a day or so to complete. The result is collapse of the main eyewall and temporary weakening of the storm. Then an outer eyewall contracts and takes its place, allowing for restrengthening. Katrina appears to have been going through the weaker stage as it approached land. The first two images capture Katrina with an intact eyewall at 5:45 p.m. Eastern Daylight Time on Sunday, August 28, as it moved over warm water in the Gulf of Mexico. By 5:45 a.m. on Monday, the weakened eyewall is being further disrupted by interaction with the land surface. Click on each image to enlarge it.

( GEMPAK images by Jeff Weber, UCAR; data from water vapor and infrared bands of NOAA GOES-E satellite .) News media terms of use * Animation of 72-hour forecast on August 27, 2005, about 60 hours before landfall These animations were created from data produced by the NCAR-based Weather Research and Forecasting model (Advanced Research WRF, or WRF-ARW). The two visualizations are based on the same 72-hour forecast of Hurricane Katrina, initialized at 0000 UTC August 27, 2005, about 60 hours before landfall. The path of the hurricane in the animation predicts almost precisely the path of the actual hurricane. Click here or on the image to open a Web page where the animations can be launched in a variety of formats.(Animations ©UCAR.) News media terms of use * About UTC: The Universal Time code is based on a 24-hour clock, with 0000 UTC equal to 7:00 p.m. Central Daylight Time. Convert other UT codes to U.S. time zones with this conversion chart from the U.S. Naval Observatory .

Question: It is common knowledge that hurricanes in the US are rated on a scale of 1 to 5. But what is the name of the scale?
Answer: Saffir-Simpson scale